Skeletal muscle metabolism in sea-acclimatized king penguins. I. Thermogenic mechanisms

Interactive Effects of Temperature Acclimation and Dietary Fatty Acids on Metabolic Rate and Body Composition of Zebra Finches (Taeniopygia guttata)

Climate change is contributing to geographic range shifts in many bird species, with possible exposure to novel diets. How individuals respond physiologically across chronic time frames to the interacting effects of diet and environmental temperature has been little explored. We acclimated zebra finches to either cool (20°C-24°C) or thermoneutral (35°C) temperatures over 6 months and provided them with diets enriched in either unsaturated or saturated fatty acids. We measured body mass throughout the study, and basal metabolic rate (BMR) and body composition at 3 and 6 months, respectively. Individuals held in cool conditions and fed a diet enriched with unsaturated fatty acids lost mass relative to the other groups, and after 6 months were of similar mass to individuals maintained at thermoneutrality. Chronic exposure to cool conditions increased BMR and the mass of the pectoral muscle and visceral organs. However, we could detect no long-term effect of diet on any physiological parameter. Our results contrast with those of birds studied over acute time frames, in which diet and temperature interact to determine energy expenditure. Over chronic time frames individuals appear to reach a new steady-state, with long-term physiological responses driven primarily by thermoregulatory responses to environmental temperature.

Serca Uncoupling May Facilitate Cold Acclimation in Dark-Eyed Juncos (Junco hyemalis) without Regulation by Sarcolipin or Phospholamban

Homeothermic endotherms defend their body temperature in cold environments using a number of behavioral and physiological mechanisms. Maintaining a stable body temperature primarily requires heat production through shivering or non-shivering thermogenesis (NST). Although the use of NST is well established in mammalian systems, the mechanisms and extent to which NST is used in birds are poorly understood. In mammals, one well-characterized mechanism of NST is through uncoupling of Ca2+ transport from ATP hydrolysis by sarco/endoplasmic reticulum ATPase (SERCA) in the skeletal muscle, which generates heat and may contribute to Ca2+ signaling for fatigue resistance and mitochondrial biogenesis. Two small proteins-sarcolipin (SLN) and phospholamban (PLN)-are known to regulate SERCA in mammals, but recent work shows inconsistent responses of SLN to cold acclimation in birds. In this study, we measured SERCA uncoupling in the pectoralis flight muscle of control (18°C) and cold-acclimated (-8°C) dark-eyed juncos (Junco hyemalis) that exhibited suppressed SLN transcription in the cold. We measured SERCA activity and Ca2+ uptake rates for the first time in cold-acclimated birds and found greater SERCA uncoupling in the muscle of juncos in the cold. However, SERCA uncoupling was not related to SLN or PLN transcription or measures of mitochondrial biogenesis. Nonetheless, SERCA uncoupling reduced an individual's risk of hypothermia in the cold. Therefore, while SERCA uncoupling in the cold could be indicative of NST, it does not appear to be mediated by known regulatory proteins in these birds. These results prompt interesting questions about the significance of SLN and PLN in birds and the role of SERCA uncoupling in response to environmental conditions.

Adipose tissue browning and thermogenesis under physiologically energetic challenges: a remodelled thermogenic system

Metabolic diseases such as obesity and diabetes are often thought to be caused by reduced energy expenditure, which poses a serious threat to human health. Cold exposure, exercise and caloric restriction have been shown to promote adipose tissue browning and thermogenesis. These physiological interventions increase energy expenditure and thus have emerged as promising strategies for mitigating metabolic disorders. However, that increased adipose tissue browning and thermogenesis elevate thermogenic consumption is not a reasonable explanation when humans and animals confront energetic challenges imposed by these interventions. In this review, we collected numerous results on adipose tissue browning and whitening and evaluated this bi-directional conversion of adipocytes from the perspective of energy homeostasis. Here, we propose a new interpretation of the role of adipose tissue browning under energetic challenges: increased adipose tissue browning and thermogenesis under energy challenge is not to enhance energy expenditure, but to reestablish a more economical thermogenic pattern to maintain the core body temperature. This can be achieved by enhancing the contribution of non-shivering thermogenesis (adipose tissue browning and thermogenesis) and lowering shivering thermogenesis and high intensity shivering. Consequently, the proportion of heat production in fat increases and that in skeletal muscle decreases, enabling skeletal muscle to devote more energy reserves to overcoming environmental stress.